Southeast Asia has the largest archipelagos and probably has the most complex geological history in the world. The overwhelming monsoon climate further promotes the extremely rich biodiversityin Southwest Asia. A large number of primitive taxa of angiosperms including Austrobaileyales and Nymphaeales were found in this region. Several tropical groups such as Cyrtandra, Aeschynanthus, and Begoniaare distributed in Southeast Asia, especially in Borneo Island and Indo-China Peninsula. Thus, Southeast Asia is not only a ‘museum’ of early angiosperms, but also acts as an ‘evolutionary front’ for some tropical taxa. Based on the plant distribution pattern, climate, and geographic history, the whole of Southeast Asia can be divided into four phytogeographic regions, i.e., Indo-China Peninsula (a part of Indo-Burma hotspot), the Sundaland (including Malay Peninsula, Borneo Island, Sumatra Island and their adjacent islands), the Wallacea (Philippines, Sulawesi Island, Java Island, Lesser Sunda Islands, Moluccas Islands), and the New Guinea. The Wallacea experiences obvious dry-wet seasonal changes in one year due to strong effects of monsoons, while Sundaland and New Guinea are wet for the whole year. Indo-Burma was connected with Sundaland during the glacial periods and acted as a ‘land bridge’ for plant dispersal in Asia, Australia, and the Pacific. Consequently, Southeast Asia became a crossroad for long-distance dispersal of modern angiosperms. Borneo and Indo-China Peninsula are the evolutionary hotspots for the whole of Southeast Asia and act as ‘species source’, while the Java Island and Lesser Sunda Islands are mainly ‘species pool’as most species on these islands are immigrants from nearby regions. For a better understanding of the species dispersal history and the formation of phytogeographic regions in Southeast Asia, the updated molecular technologies should be used, and the plant taxa should be sampled in the entire distribution range in future studies.